How do the AV-sync, Frame Dropping and No sync in Timeline work?
They come in help when the animation is too heavy to handle realtime reproduction, affecting only the 3D viewport, not the renders.

No sync means that every frame in the viewport is showed as soon as possible, and a number in the upper left corner of the 3D view tells you the frame rate actually being reproduced: if white everything is OK (I.E. 24 FPS - default), if Red is slower (Maybe 4.67 FPS).

Frame Dropping means that the FPS is always the choosen one (24 - default), and blender jumps from a frame to another trying to remain in sync, actually not displaying (dropping) all the frames between.

AV Sync is used in combination with the VSE, assuring corrispondence between the 3D frame and the VSE frame.